1. Field of the Invention
The present invention relates to a magnetic recording medium and a method for manufacturing the same. More specifically, the magnetic recording medium has an extremely thin magnetic layer having a thickness of at most 0.3 .mu.m.
2. Description of the Related Art
In recent years, in the field of magnetic recording media, recording wavelengths have been shortened and recording tracks have been narrowed in order to achieve high-density recording. Accordingly, improvements of output characteristics, signal-to-noise ratio (S/N) and overwrite characteristics have been required. In the magnetic recording media for high-density recording, in order to improve the output characteristics and the S/N, ferromagnetic metal powder has been made finer, given a higher magnetization and a higher magnetic coercive force and mixed into magnetic paints at a higher density. Further, the surface of the magnetic recording media requires a smoother finish. In order to improve the overwrite characteristics, the magnetic layer has been made thinner. Recently, these characteristics have been significantly improved by adopting a multi layer structure which has a lower nonmagnetic layer provided on a flexible nonmagnetic support, and an upper magnetic layer provided on the lower nonmagnetic layer.
When a conventional contact type magnetic head is used, the friction between the magnetic head and the surface of the upper magnetic layer of the recording medium increases as the surface of the upper magnetic layer is increasingly smoother. Thus the running characteristics deteriorate. On the other hand, in order to improve the running characteristics, if the surface of the upper magnetic layer is roughened, the S/N deteriorates. Accordingly, the running characteristics and electromagnetic performance must be balanced.
Magnetic recording media are disclosed in Japanese Unexamined Patent Publication (Kokai) 5-73,883 (hereinafter referred to as the " 883 kokai"), entitled "Magnetic Recording Medium", Japanese Unexamined Patent Publication (Kokai) 5-298,653 (hereinafter referred to as the " 653 kokai"), entitled "Magnetic Recording Medium", Japanese Unexamined Patent Publication (Kokai) 9-320,037 (hereinafter referred to as the " 037 kokai"), entitled "Magnetic Recording Medium", Japanese Unexamined Patent Publication (Kokai) 3-17,816 (hereinafter referred to as the " 816 kokai"), entitled "Magnetic Recording Medium", and Japanese Unexamined Patent Publication (Kokai) 3-224,127 (hereinafter referred to as the " 127 kokai"), entitled "Magnetic Recording Medium". The contents of these applications are incorporated herein by reference in their entirety.
In the magnetic recording media disclosed in the 883 kokai, the 653 kokai and the 037 kokai, a lower nonmagnetic layer is provided on a nonmagnetic support. The nonmagnetic layer includes carbon black and a binder. An upper magnetic layer is provided on the nonmagnetic layer.
In the magnetic recording media disclosed in the 816 kokai, a magnetic layer is provided on a nonmagnetic support. The magnetic layer includes magnetic powder and a binder. The surface roughness of the magnetic layer is limited such that the ratio Ns/Ns(t) is between 0.10 and 0.35, wherein Ns represents a number of spikes which project 0.01 .mu.m or more from an average line of a surface roughness sectional curve, and Ns(t) represents a total number of spikes which project from the average line of the surface roughness sectional curve.
In the magnetic recording medium disclosed in the 127 kokai, a magnetic layer is coated on a nonmagnetic support. A number of peaks on the surface of the magnetic layer is limited to 100 or more.
A nonmagnetic coating solution for a nonmagnetic layer provided on a flexible nonmagnetic support includes carbon in order that the nonmagnetic layer has good conductivity. Generally, the solution which includes carbon has thixotropy. Accordingly, because of the thixotropy, a problem arises in that the surface projections on the flexible nonmagnetic support project from the nonmagnetic layer even when the nonmagnetic coating solution is coated on the flexible nonmagnetic support and covers the surface projections. Therefore, the surface projections on the flexible nonmagnetic support form projections on the surface of the lower nonmagnetic layer. Thus, since the surface characteristics of the upper magnetic layer significantly deteriorate, friction characteristics, durability and electromagnetic characteristics deteriorate.